Check valves are normally installed in air supply lines to plenum chambers used in fluidized unloading systems. One such check valve is shown at 84 in U.S. Pat. No. 3,708,209 assigned to the same assignee as the present application. Such check valves are positioned to prevent the back flow of air through the air supply if the blower is shut off. This back flow can pressurize the manifold causing a safety hazard if an attendant attempts to disconnect the air inlet or the product discharge line. It can also cause damage to the flower by causing it to rotate in the reverse direction.
The check valves normally used are installed between mounting flanges in the air supply line. See FIG. 6 of U.S. Pat. No. 3,708,209. These valves normally include a spring loaded disc which can be become jammed or can become inoperative if the spring breaks. They must also be fabricated from expensive materials such as stainless steel to prevent corrosion.
Flow restriction is desirable for safety reasons. In the event a primary check valve fails, for example, by breaking of its biasing spring, it is desirable that flow from a plenum chamber or pressurized container will be dissipated slowly, over time, and not rapidly which constitutes a safety hazard to attendants working around the container.